Conventionally, in order to prevent a connector from being semi-fitted, i.e., a method of facilitating the coupling operation itself of the connector by, e.g., decreasing an inserting force required for inserting one connector in the other connector or by providing a mechanism for multiplying a force to be applied to the connector during insertion, a method of visually checking a semi-fitted state, and a method of electrically detecting the conductive states of the contacts are known.
However, when the inserting force for the connector is decreased, the coupled connectors tend to be easily disconnected. The method of multiplying the force to be applied during insertion or detecting the conductive states of the contacts complicates the entire structure or requires a special device or measuring tool.